Polyethylene Glycol-induced Water Stress Research Articles

Morphological and Physio-Chemical Responses to PEG-Induced Water Stress in Vanilla planifolia and V. pompona Hybrids

Vanilla planifolia is an orchid of cultural and economic value. However, its cultivation in many tropical countries is threatened by water stress. In contrast, V. pompona is a species that is tolerant of prolonged periods of drought. Due to the need for plants' resistant to water stress, the use of hybrids of these two species is considered. Therefore, the objective of this study was to evaluate the morphological and physio-chemical responses of in vitro vanilla seedlings of the parental genotype V. planifolia, and the hybrids V. planifolia × V. pompona and V. pompona × V. planifolia, which were then exposed over five weeks to polyethylene glycol-induced water stress (-0.49 mPa). Stem and root length, relative growth rate, number of leaves and roots, stomatal conductance, specific leaf area, and leaf water content were determined. Metabolites potentially associated with the response to water stress were identified in leaves, through untargeted and targeted metabolomics. Both hybrids exhibited a smaller decrease in the morphophysiological responses compared to V. planifolia and exhibited an enrichment of metabolites such as carbohydrates, amino acids, purines, phenols, and organic acids. Hybrids of these two species are considered as a potential alternative to the traditional cultivation of vanilla to face drought in a global warming scenario.

Screening of Pearl Millet Genotypes Suitable for Drought Tolerance at Early Seedling Stage

Background: Pearl millet [Pennisetum glaucum (L.) Br.] is the most widely grown staple food of majority of poor and small land holders in Asia and Africa. It is also consumed as feed and fodder for livestock. It is the sixth most important cereal crop in the world next to maize, rice wheat, barley and sorghum. The temperature is one of the key climatic factors and has profound effect on the growth and development of the pearl millet. It can only be managed through developing hybrid varieties which can tolerate high temperature during germination and early seedling stages.The present study aimed to identify drought tolerant genotypes of pearl millet at seedling stage. Methods: This experiment was carried out at Mandor during kharif 2018 with five selected pearl millet advanced hybrids MH 2192, MH 2224, MH 2228, MH 2354 and MH 2359 along with three checks RHB 177, MPMH 17 and 86M86 which were tested under polyethylene glycol (PEG) (5% and 10%) induced osmotic stress. Various physiological parameters were recorded 15 days after sowing and statistical analysis made using Windostat software. Result: The results revealed that shoot length, seedling dry weight, relative water content, membrane stability index and chlorophyll content decreased significantly with PEG induced water stress in all the hybrids while root length and catalase activity increased significantly under water stress. Among 5 hybrids, two hybrids viz. MH 2359 and MH 2354 performed better and found to be superior under PEG induced water stress. Thus, various drought tolerance indices may further be studied for these two hybrids and can be used in development of drought tolerant genotypes which may prove helpful for crop improvement programs of pearl millet.

In Vitro Screening of Sugarcane Cultivars (Saccharum spp. Hybrids) for Tolerance to Polyethylene Glycol-Induced Water Stress

Water stress caused by drought affects the productivity of the sugarcane crop. A breeding alternative is the selection of drought-tolerant sugarcane cultivars. The objective of this study was the in vitro screening of cultivars tolerant to water stress using polyethylene glycol (PEG) as a stressing agent. Cultivars (cv) Mex 69-290, CP 72-2086, Mex 79-431 and MOTZMex 92-207 were subjected to different concentrations of PEG 6000 (0, 3, 6 and 9% w/v) using Murashige and Skoog semi-solid culture medium. At 30 days of culture, different developmental variables and dry matter (DM), total protein (TP), proline (Pr) and glycine-betaine (GB) contents were evaluated. The results showed reduced development in cv CP 72-2086, Mex 79-431 and MOTZMex 92-207 with increasing PEG concentration. The cv Mex 69-290 showed tolerance to osmotic stress of −0.45 MPa using 3% PEG. Overall, TP content decreased with increasing PEG concentration, while DM, Pr and GB contents rose with increasing PEG concentration in all evaluated cultivars. Our results suggest that cv Mex 69-290 has a slight tolerance to water stress and could be used for rainfed cultivation with low rainfall or reduced irrigation for better water use efficiency. In conclusion, the in vitro screening technique of cultivars tolerant to PEG-induced water stress is an alternative for early determination of drought stress in sugarcane.

Assessment of biochemical and physiological parameters of durum wheat genotypes at the seedling stage during polyethylene glycol-induced water stress

Wild and landrace progenitors of wheat are an important source of resistance to various types of environmental stress, which can be explored in wheat breeding. With the aim of providing genetic materials for breeding to generate drought-resistant varieties, 50 durum wheat landraces, breeding lines and local cultivars were evaluated at the seedling stage for biomass, physiological parameters (e.g. photosynthetic pigment and relative water contents) and activities of key enzymatic antioxidants [e.g. ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD)] under optimal and polyethylene glycol (PEG)-triggered water stress. We observed significant differences in the measured parameters among the durum wheat genotypes in response to PEG-induced water stress. Biomass and physiological traits showed a decreasing trend, whereas the antioxidant enzyme activities displayed an increasing trend in the investigated durum wheats under PEG-induced water stress. PEG-induced water stress decreased the average means of total soluble protein, chlorophyll a and total chlorophyll contents of the 50 investigated genotypes by 51.42, 36.77 and 26.15%, respectively, while the mean values of POD and SOD activities increased by 173.33 and 92.26%, respectively, relative to the control. A principal component analysis (PCA)-based biplot demonstrated that the stress tolerance index (STI) positively correlated with the root and shoot dry weights as well as CAT, SOD and APX activities. In conclusion, several breeding lines (e.g. G10, G19 and G20), Iranian landraces (e.g. G42 and G47) and local cultivar (G49) showed a better adaptive response to PEG-induced water stress than other tested genotypes, suggesting that these genotypes could be explored in various breeding programs to create new wheat cultivars with beneficial water stress-adaptive traits.

Assessing in vitro germination and seedling growth of tomato (<em>Solanum lycopersicum</em> L.) cv KC-1 in response to polyethylene glycol-induced water stress

Polyethylene glycol (PEG) is a high molecular weight osmoticum, which can be used to induce water stress artificially in a nutrient medium. An experiment was conducted with four concentrations of polyethylene glycol namely 30 g/L, 60 g/L, 90 g/L with a control to study the effect of PEG on seed germination and seedling performance of tomato cv. KC-1. The experiment was laid out on a complete randomized design (CRD) with six replicates. Data was analyzed using the regression and Pearson correlation analysis. The PEG concentrations fortified to the culture media had significant negative relationships (p<0.0001) for all estimated parameters such as shoot length, root length, fresh weight, dry weight, chlorophyll a, chlorophyll b, total chlorophyll content of the in vitro grown tomato seedlings. The MS media fortified with 0 mg/L PEG treatment (control) exhibited better seed germination percentage (87.8%) and seedling performance, while the minimum value of seed germination (21.9%) was noted under 90 g/L PEG treatment at 20 days after culture. In control PEG treatment, high values were noted for shoot length (2.88 cm), root length (1.67 cm), fresh weight (60.37 mg), dry weight (8.93 mg), chlorophyll a (1.38 mg/g), chlorophyll b (0.48 mg/g), total chlorophyll content (1.86 mg/g) after 4 weeks of culture. The decreased in seed germination and seedling growth rate probably due to lower the water potential of the culture media by supplement with the PEG into the MS medium fortified with 2 mg/L BAP. In vitro germination and seedling growth were considerably reduced on 60 g/L and 90 g/L PEG concentrations.

Water deficit-dependent changes in non-structural carbohydrate profiles, growth and mortality of pine and spruce seedlings in hydroculture

Sugars, sugar alcohols and starch are the main components of the mobile carbon pool in plants. Water stress substantially affects the carbon balance in plants and the reserves of mobile carbon and can lead to their exhaustion and the development of carbon starvation. In addition to their function as reserve compounds, many sugars and sugar alcohols perform specific protective functions under water stress, acting as osmotically active compounds, chemical chaperones, and ROS scavengers. Pinus sylvestris and Picea abies are isohydric species that are prone to substantial changes in carbon balance under water stress. Therefore, we analysed the dynamics of sugars, sugar alcohols and starch in seedlings of these two species under polyethylene glycol-induced water stress ranging from relatively weak (culture medium water potential of −0.15 MPa) to very strong (−1.0 MPa) intensities. The water stress throughout the intensity range significantly increased the mortality of the seedlings of both species, and the extent of mortality was not correlated with the degree of seedling dehydration. The greatest mortality of pine seedlings was observed under −0.5 MPa and coincided with a sharp (greater than 60%) decrease in the total pool of carbohydrates and sugar alcohols in the roots. We assume that the main cause of death of pine seedlings under such conditions was carbon starvation of the roots. The most likely causes of carbon reserve depletion in the pine roots were the rapid root growth during the initial period of water stress in combination with the disruption of the assimilate transport from the shoots, as mobile carbon reserves increased substantially in the pine needles under water stress. Unlike those of pine, changes in the mobile carbon pools in both the roots and needles of spruce did not correlate with seedling mortality. Therefore, carbon starvation was likely not the reason for spruce seedling death under water stress. Although severe water stress often depletes starch pools in plants, we observed an increase in starch accumulation in both species under water stress. In pine, this increase could be due to the inhibition of starch hydrolysis arising from a significant increase in the content of maltose under water stress. Based on an analysis of the dynamics of individual sugars and sugar alcohols under water stress, we can assume that raffinose and non-cyclic sugar alcohols play a role in the response of both species to water stress.

Water stress alleviation by polyamines and phenolic compounds in Scrophularia striata is mediated by NO and H2O2

Plants respond to water stress through a variety of mechanisms, depending on metabolites preferences and their available resources. This work was performed to elucidate the cross-talk between signaling molecules (polyamines (PAs), hydrogen peroxide (H2O2) and nitric oxide (NO)), phenolic compounds and osmolytes (phenylethanoid glycosides (PhGs), phenolic acids, flavonoids, soluble sugars and amino acids) under water stress in Scrophularia striata plants. The results revealed that PAs, NO levels were enhanced in the plants, earlier in response to polyethylene glycol-induced water stress. The antioxidative mechanisms with increased activity of catalase (CAT), guaiacol peroxidase (GPX) and superoxide dismutase (SOD) and also phenylalanine ammonia-lyase (PAL), tyrosine ammonia-lyase (TAL), as key enzymes in phenolic pathway were deployed in response to the stress. Mannose, glucose, xylose/rhamnose which are involved in PhGs biosynthesis as well as in serving osmotic adjustment were modulated. The elevated content of arginine and methionine as PAs precursors and tyrosine and phenylalanine as PhGs precursors was enhanced by water stress and was significantly associated with PAs and PhGs accumulations. Metabolic profiling revealed new information about relationship between stress signal molecules; PAs, NO and H2O2, osmolytes (sugers, PhGs) and phenolic compounds which involved in the improvement of water stress tolerance in S. striata.

Priming effect on the physiological potential of maize seeds under abiotic stress1

ABSTRACT Abiotic stress directly influences seed performance, so poor-vigor seeds under adverse conditions tend to show lower germination speed and rate. By controlling the hydration level of seeds (i.e., priming) with the addition of chemical agents, it is possible to elicit the maximum physiological potential of seeds, even under stress conditions. This study aimed to evaluate the priming effect of different chemical agents on the physiological potential of maize seeds under abiotic stress (polyethylene glycol induced water stress, hypoxia, low temperature and salt stress after controlled deterioration). The experimental design was completely randomized, in a 5 x 2 factorial scheme, consisting of different chemical agents used to hydrate the seeds [control without priming; control primed with water; calcium nitrate (0.2 %); amino acid L-phenylalanine (0.05 %); amino acid L-phenylalanine (0.5 %) + calcium nitrate (0.2 %)] and 2 seed lots, with four replicates. Priming with calcium nitrate lead to a greater germination and higher emergence rate of the seedlings under suboptimal temperature conditions, and seeds that underwent controlled deterioration showed greater germination levels with the use of calcium nitrate + phenylalanine in the priming process, regardless of the lot used. In general, seed priming allowed a greater expression of seed vigor, even though an interaction with lots was observed in some variables.

Identification of PEG-induced water stress responsive transcripts using co-expression network in Eucalyptus grandis

Ecophysiological studies in Eucalyptus have shown that water is the principal factor limiting stem growth. Effect of water deficit conditions on physiological and biochemical parameters has been extensively reported in Eucalyptus. The present study was conducted to identify major polyethylene glycol induced water stress responsive transcripts in Eucalyptus grandis using gene co-expression network. A customized array representing 3359 water stress responsive genes was designed to document their expression in leaves of E. grandis cuttings subjected to −0.225MPa of PEG treatment. The differentially expressed transcripts were documented and significantly co-expressed transcripts were used for construction of network. The co-expression network was constructed with 915 nodes and 3454 edges with degree ranging from 2 to 45. Ninety four GO categories and 117 functional pathways were identified in the network. MCODE analysis generated 27 modules and module 6 with 479 nodes and 1005 edges was identified as the biologically relevant network. The major water responsive transcripts represented in the module included dehydrin, osmotin, LEA protein, expansin, arabinogalactans, heat shock proteins, major facilitator proteins, ARM repeat proteins, raffinose synthase, tonoplast intrinsic protein and transcription factors like DREB2A, ARF9, AGL24, UNE12, WLIM1 and MYB66, MYB70, MYB 55, MYB 16 and MYB 103. The coordinated analysis of gene expression patterns and coexpression networks developed in this study identified an array of transcripts that may regulate PEG induced water stress responses in E. grandis.

Antioxidant system and photosynthetic characteristics responses to short-term PEG-induced drought stress in cucumber seedling leaves

The effect of short-term drought stress on the water content, antioxidant system and photosynthetic characteristics was investigated using cucumber (Cucumis sativus L.) seedlings. The results indicated that polyethylene glycol induced water stress reduced water content in shoots of cucumber seedling after treatment of 36 hours, and caused obvious reductions in net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration of leaves. In addition, water stress significantly reduced the photosynthetic pigment content and inhibited photochemical activity, including actual photochemical efficiency, maximal quantum yield of photosystem II photochemistry and coefficient for photochemical quenching. Meanwhile non-photochemical quenching increased. As responses to drought stress, significant increases in electrolyte leakage, malondialdehyde, superoxide anion and hydrogen peroxide levels were detected in leaves. The superoxide dismutases, catalase, glutathione reductase and dehydroascorbate reductase activities, protein, ascorbate and glutathione content, all decreased and peroxidases activity increased, while ascorbate peroxidase and monodehydroascorbate reductase activities exhibited different trend under different degree of water stress. Therefore, it can be concluded that water stress strongly disrupted the normal metabolism of leaves and restrained water absorption.

Involvement of 14-3-3 protein GRF9 in root growth and response under polyethylene glycol-induced water stress.

Plant 14-3-3 proteins are phosphoserine-binding proteins that regulate a wide array of targets via direct protein-protein interactions. In this study, the role of a 14-3-3 protein, GRF9, in plant response to water stress was investigated. Arabidopsis wild-type, GRF9-deficient mutant (grf9), and GRF9-overexpressing (OE) plants were treated with polyethylene glycol (PEG) to induce mild water stress. OE plant showed better whole-plant growth and root growth than the wild type under normal or water stress conditions while the grf9 mutant showed worse growth. In OE plants, GRF9 favours the allocation of shoot carbon to roots. In addition, GRF9 enhanced proton extrusion, mainly in the root elongation zone and root hair zone, and maintained root growth under mild water stress. Grafting among the wild type, OE, and grf9 plants showed that when OE plants were used as the scion and GRF9 was overexpressed in the shoot, it enhanced sucrose transport into the root, and when OE plants were used as rootstock and GRF9 was overexpressed in the root, it caused more release of protons into the root surface under water stress. Taken together, the results suggest that under PEG-induced water stress, GRF9 is involved in allocating more carbon from the shoot to the root and enhancing proton secretion in the root growing zone, and this process is important for root response to mild water stress.

Osmotin-expressing transgenic tea plants have improved stress tolerance and are of higher quality

Drought is a major stress that affects the yield and quality of tea, a widely consumed beverage crop grown in more than 20 countries of the world. Therefore, osmotin gene-expressing transgenic tea plants produced using earlier optimized conditions were evaluated for their tolerance of drought stress and their quality. Improved tolerance of polyethylene glycol-induced water stress and faster recovery from stress were evident in transgenic lines compared with the normal phenotype. Significant improvements in growth under in-vitro conditions were also observed. Besides enhanced reactive oxygen species-scavenging enzyme activity, the transgenic lines contained significantly higher levels of flavan-3-ols and caffeine, key compounds that govern quality and commercial yield of the beverage. The selected transgenic lines have the potential to meet the demands of the tea industry for stress-tolerant plants with higher yield and quality. These traits of the transgenic lines can be effectively maintained for generations because tea is commercially cultivated through vegetative propagation only.

Polyethylene glycol induced water stress in maize seedlings and evaluation of antioxidant defense mechanisms

Maize is one such crop, the production of which is highly challenged due to water shortage and soil water losses. The present study was undertaken on artificially induced water stress of maize in vitro, where stress was applied with PEG-6000 on one week old seedlings of four varieties BN 10, Dhanya. Kaveri-Super 244, and Swarna for 3, 5 and 7 days. The activity of antioxidative enzyme like peroxidase, catalase, ascorbate peroxidase, glutathione reductase and superoxide dismutase was assayed in the stressed and control plants. Peroxidase activity decreased on the 7th day in Dhanya and Swarna but in BN 101 and Super 244 the activity decreased slightly on the 5th day and increased again on the 7th day. Ascorbate peroxidase and superoxide dismutase showed a similar trend where the activity decreased after a certain period of stress. Similar trend was seen for GR activity too in case of Dhanya and Swarna. But in BN 101 and Super 244 there was an increase in the activity with the increase in the period of stress. Catalase activity declined during stress in Dhanya and Swarna while the other two varieties showed an increase during stress. Other than enzymatic activities, various biochemical analyses like proline, ascorbate, chlorophyll was also carried out. With the increase in intensity of drought there was an increase in both proline and ascorbate content in all. A significant increase in the ascorbate content was observed in BN 101 and super 244, H202, accumulation and lipid peroxidation showed an increase during stress in Dhanya and Swarna but no increase was seen in the other two varieties. Chlorophyll content showed a decline during the period of drought when compared to the control plants of all varieties. Enzymatic activity and biochemical tests show that Dhanya and Swarna are susceptible to drought stress than super 244 and BN 101 which are the tolerant varieties.

Enhancement of superoxide dismutase activity in the leaves of white clover (Trifolium repens L.) in response to polyethylene glycol-induced water stress

Effects of polyethylene glycol (PEG)-induced water stress on the activities of total leaf superoxide dismutase (SOD) and chloroplast SOD (including thylakoid-bound SOD and stroma SOD) are described in white clover (Trifolium repens L.) grown in solution culture from rooted cuttings. Both leaf SOD and chloroplast SOD activities were markedly enhanced with increasing concentration of PEG stress, generating osmotic potentials around the roots 0, −0.5, −1.0, −1.5 MPa. The effects increased with time up to 72 h. Chloroplast Fe-containing SOD represented about 30% of the total leaf SOD activity in the control plants and a significant increase in chloroplast SOD activity was found during the stress period. This accounted for about 35.5–71.1% of the total leaf SOD activity. The proportion of chloroplast SOD in total leaf SOD not only increased with the decreasing of osmotic potential, but also increased with incubation time. Furthermore, the increase in thylakoid-bound SOD activity was much higher than that of stroma SOD in chloroplast of plants under water stress. The enhanced chloroplastic SOD activity, especially thylakoid-bound SOD activity, demonstrated in Trifolium repens suggests that Fe-SOD located in chloroplasts play a more important role than cytosolic Cu/Zn-containing SODs in scavenging O2−.

Differential Sensitivity of Macrocarpa and Microcarpa Types of Chickpea (Cicer arietinum L.) to Water Stress: Association of Contrasting Stress Response with Oxidative Injury

Chickpea (Cicer arietinum L.) is particularly sensitive to water stress at its reproductive phase and, under conditions of water stress, will abort flowers and pods, thus reducing yield potential. There are two types of chickpea: (i) Macrocarpa (“Kabuli”), which has large, rams head-shaped, light brown seeds; and (ii) Microcarpa (“Desi”), which has small, angular and dark-brown seeds. Relatively speaking, “Kabuli” has been reported to be more sensitive to water stress than “Desi”. The underlying mechanisms associated with contrasting sensitivity to water stress at the metabolic level are not well understood. We hypothesized that one of the reasons for contrasting water stress sensitivity in the two types of chickpea may be a variation in oxidative injury. In the present study, plants of both types were water stressed at the reproductive stage for 14 d. As a result of the stress, the “Kabuli” type exhibited an 80% reduction in seed yield over control compared with a 64% reduction observed for the “Desi” type. The decrease in leaf water potential (Ψw) was faster in the “Kabuli” compared with the “Desi” type. At the end of the water stress period, Ψw was reduced to −2.9 and −3.1 MPa in the “Desi” and “Kabuli” types, respectively, without any significant difference between them. On the last day of stress, “Kabuli” experienced 20% more membrane injury than “Desi”. The chlorophyll content and photosynthetic rate were significantly greater in “Desi” compared with “Kabuli”. The malondialdehyde and H2O2 content were markedly higher at the end of the water stress in “Kabuli” compared with “Desi”, indicating greater oxidative stress in the former. Levels of anti-oxidants, such as ascorbic acid and glutathione, were significantly higher in “Desi” than “Kabuli”. Superoxide dismutase and catalase activity did not differ significantly between the two types of chickpea, whereas on the 10th day, the activities of ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase were higher in “Desi”. These findings indicate that the greater stress tolerance in the “Desi” type may be ascribed to its superior ability to maintain better water status, which results in less oxidative damage. In addition, laboratory studies conducted by subjecting both types of chickpea to similar levels of polyethylene glycol-induced water stress and to 10 μ.mol/L abscisic acid indicated a greater capacity of the “Desi” type to deal with oxidative stress than the “Kabuli” type.

Impact of polyethylene glycol-induced water stress on growth and development of shoot tip cultures from different banana (Musa spp.) cultivars

Impact of polyethylene glycol-induced water stress on growth and development of shoot tip cultures from different banana (Musa spp.) cultivars

Differential response of two Vicia faba cultivars to drought: Growth, pigments, lipid peroxidation, organic solutes, catalase and peroxidase activity

A study on the germination of five Vicia faba cultivars exposed to polyethylene glycol-induced water stress indicated that cv Giza 40 showed the highest germination capacity and cv Giza 667 the lowest. The effect of low soil water content was studied on the plant growth, photosynthetic pigment content, organic solutes, relative water content (RWC), lipid peroxidation, membrane stability index (MSI), and the catalase (CAT) and peroxidase (POX) activity in the leaves of 21-day-old Vicia faba cv Giza 40 and cv Giza 667 plants. With respect to dry weight (DW), drought caused a greater decrease in cv Giza 667 than in cv Giza 40, indicating that cv Giza 40 was more tolerant of low soil water content. Drought decreased the Chl a, Chl b and carotenoid contents and the Chl a/b and carotenoid/Chl a+b ratios in the leaves of cv Giza 667, while in cv Giza 40 a significant increase in these pigment parameters was observed under drought stress. Drought caused a decrease in RWC and MSI and an increase in the lipid peroxidation level and in the catalase (CAT) and peroxidase (POX) activity in both the cultivars, but the decline in RWC and MSI and the increase in lipid peroxidation level in response to drought stress were greater in cv Giza 667 than in cv Giza 40. The CAT and POX activities were higher in Giza 40 than in Giza 667 under both control and drought conditions. Drought induced the accumulation of soluble sugars, soluble proteins, free amino acids and proline in both cultivars. However, this accumulation was lower in cv Giza 667 than in the more tolerant cv Giza 40. These results indicate that cv Giza 40 showed better protection against drought-induced oxidative stress through higher CAT and POX activities and osmolyte concentrations than cv Giza 667.

Differential responses of superoxide dismutase in freezing resistant Solanum curtilobum and freezing sensitive Solanum tuberosum subjected to oxidative and water stress

In photosynthetic tissues superoxide dismutase (SOD) plays an important role by scavenging the superoxide radical whose production is an usual reaction in chloroplast thylakoids. To test the differential response of SOD, two Andean potato species differing in frost resistance, Solanum curtilobum (frost resistant) and Solanum tuberosum (frost sensitive), were subjected to methyl viologen-mediated oxidative stress and polyethylene glycol (PEG)-induced water stress. A significant increment (approximately two-fold) in total SOD and FeSOD activity, which occupied about 50% of the total activity, was found when leaves of S. curtilobum were exposed to water stress. In contrast, the SOD activity in leaves of S. tuberosum remained unchanged. The exposure of leaves of S. curtilobum to oxidative stress increased total SOD and FeSOD activity by 350%. High correlation between SOD activity and the F v/ F m ratio under both PEG induced water stress and MV-mediated oxidative stress was observed. This suggests that SOD can protect PSII from superoxide generated by oxidative and water stress. The higher SOD activity could be an important mechanism to explain why some natives Andean potato like S. curtilobum are more resistant to abiotic stresses than S. tuberosum.

Ferricyanide restores nitrate reductase activity in leaf extracts of water stressed wheat seedlings

Summary Nitrate reductase (EC 1.6.6.1) activity declined rapidly in leaves of wheat seedlings following polyethylene glycol-induced water stress. The decrease in NADH-nitrate reductase was accompanied by a concomitant loss of FMNH 2 − and reduced benzyl viologen-linked nitrate reductase activities. Incubation of dialysed leaf extracts with potassium ferricyanide restored the enzyme activity almost to the same level as that in non-stressed plants. The decline in nitrate reductase, at least during the initial stages of water stress, hence seems to be primarily due to redox-mediated inactivation rather than diminished synthesis of the enzyme.

Changes in cell ultrastructure and endogenous abscisic acid and indole-3-actic acid concentrations in Fatsia japonica leaves under polyethylene glycol-induced water stress

Ultrastructural alterations in epidermal and mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in PEG-treated plants of Fatsia japonica Decne & Plank. Under stress induced by PEG vesicles containing fibrous material and electron-dense bodies associated with plasma membranes were observed. Cytochemical examination indicated that electron-dense bodies corresponded to lipids and the fibrous material of the vesicles were polysaccharides. Chloroplasts, mitochondria, nuclei and Golgi apparatus also showed modifications. A strong relationship was found between increasing PEG-induced water stress, increasing endogenous ABA and ultrastructural changes. In relation with leaf ontogeny and ABA concentration a higher ABA level was observed in younger than in older leaves. The differences in the endogenous concentrations of indole-3-acetic acid are unclear, except after 7 days of PEG-treatment. The increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.